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Retirement Plans and Other Postretirement Benefits | 9 Months Ended
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Sep. 30, 2023
Retirement Benefits [Abstract] |
Retirement Plans and Other Postretirement Benefits | Retirement Plans and Other Postretirement Benefits Pinnacle West sponsors a qualified defined benefit and account balance pension plan, a non-qualified supplemental excess benefit retirement plan, and other postretirement benefit plans for the employees of Pinnacle West and our subsidiaries. The other postretirement benefit plans include a group life and medical plan and a post-65 retiree health reimbursement arrangement (“HRA”). Pinnacle West uses a December 31 measurement date each year for its pension and other postretirement benefit plans. The market-related value of our plan assets is their fair value at the measurement date. The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):
| Pension Benefits |  | Other Benefits
| Three Months Ended September 30, |  | Nine Months Ended September 30, |  | Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Service cost — benefits earned during the period | $ | 9,865 |  |  | $ | 13,868 |  |  | $ | 29,595 |  |  | $ | 41,605 |  |  | $ | 2,142 |  |  | $ | 4,117 |  |  | $ | 6,426 |  |  | $ | 12,352 |
Non-service costs (credits): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest cost on benefit obligation | 38,390 |  |  | 26,873 |  |  | 115,170 |  |  | 80,619 |  |  | 5,627 |  |  | 4,372 |  |  | 16,882 |  |  | 13,118 |
Expected return on plan assets | (45,735) |  |  | (46,443) |  |  | (137,204) |  |  | (139,331) |  |  | (10,872) |  |  | (11,510) |  |  | (32,616) |  |  | (34,531) |
Amortization of: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prior service credit | — |  |  | — |  |  | — |  |  | — |  |  | (9,447) |  |  | (9,447) |  |  | (28,341) |  |  | (28,341) |
Net actuarial loss/(gain) | 9,605 |  |  | 4,379 |  |  | 28,815 |  |  | 13,136 |  |  | (2,404) |  |  | (3,209) |  |  | (7,211) |  |  | (9,627) |
Net periodic cost/(benefit) | $ | 12,125 |  |  | $ | (1,323) |  |  | $ | 36,376 |  |  | $ | (3,971) |  |  | $ | (14,954) |  |  | $ | (15,677) |  |  | $ | (44,860) |  |  | $ | (47,029) |
Portion of cost/(benefit) charged to expense | $ | 6,828 |  |  | $ | (4,246) |  |  | $ | 20,568 |  |  | $ | (12,258) |  |  | $ | (10,851) |  |  | $ | (11,318) |  |  | $ | (32,536) |  |  | $ | (33,736) |
Contributions We have not made any voluntary contributions to our pension plan year-to-date in 2023. The minimum required contributions for the pension plan are zero for the next three years and we do not expect to make any contributions in 2023, 2024 or 2025. With regard to contributions to our other postretirement benefit plan, we have not made a contribution year-to-date in 2023 and do not expect to make any contributions in 2023, 2024 or 2025.

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